A computerized tomography (CT) scan acquires the radiodensity, also termed radiopacity, of the scanned portion of anatomy. Radiodensity is measured in Hounsfield units (HU), with air having HU=−1000, water having HU=0, and cortical bone having HU=+3000. In a CT image, the acquired radiodensity values are mapped to different grayscale values. Typically, in a CT image, air is presented in black, cortical bone in white, and other materials in varying shades of gray.
Traditionally, interventional radiologists have been trained to navigate the head of a subject using two-dimensional (2D) images of the head. For example, during a sinus procedure, the interventional radiologist may refer to three computerized tomography (CT) slices of the subject's head: an axial slice, a coronal slice, and a sagittal slice.
U.S. Pat. No. 8,532,738, whose disclosure is incorporated herein by reference, describes a method, including constructing a simulated surface of a body cavity, and pressing a distal end of a probe against a wall of the body cavity. While pressing the distal end against the wall, position measurements are accepted from the probe indicating a position of the probe within the body cavity, and force measurements are accepted from the probe indicating a force between the distal end and the wall. A distortion in the simulated surface is created at the position indicated by the position measurements, so as to form a distorted surface, upon detecting that the force measurements exceed a predefined amount. The distorted surface is then displayed.
U.S. Pat. No. 7,924,279, whose disclosure is incorporated herein by reference, describes a system for visualizing a 3D volume, in particular for medical applications, that includes an input for receiving a three-dimensional set of data representing voxel values of the 3D volume. The data set is stored in a storage. A processor projects the volume onto an imaginary 2D projection screen from a predetermined viewpoint. For each pixel of the 2D projection image a ray is cast through the pixel and through the volume. A protocol is used that, while traversing along ray positions within the volume, determines a rendering algorithm and/or rendering parameters in dependence on the ray position. For each ray position the determined rendering algorithm/parameters are used to calculate a contribution to a pixel value of the pixel based on at least one voxel value within a predetermined range of the ray position. An output is used for providing pixel values of a 2D image for rendering on a display.